The present invention relates to two piece, swage type fasteners including a pin and collar with the collar adapted to be swaged into lock grooves in the pin. Swage fasteners can be of a pull type or a non-pull stump type. Swage fasteners of the pull type include a pin with a pintail section having pull grooves adapted to be gripped by an installation tool and severed at a breakneck groove after completion of swage. The present invention is directed to a fastener having a unique construction in which the loads required for collar swage have been decreased. This also permits a reduction in the load required to sever the pintail section. At the same time, the construction of the present invention also provides a fastener of an optimized design having a preselected strength and at a minimized installed weight similar to existing pull type swage fasteners of optimized design but which require higher swage and pintail severance loads. The low swage load construction is also advantageous with non-pull type or stump type swage fasteners.
In the past, such optimized swage type fasteners were made of a unique design with significance given to strength and to the minimization of weight. These fasteners were especially directed for use in the aerospace industry where weight is an important factor. Such a swage type fastener of optimized design is shown and described in U.S. Pat. No. 4,472,096 to J. Ruhl and R.
Dixon issued Sept. 18, 1984. As will be seen the present invention is related to the concepts of the '096 patent and, like the fasteners of the '096 patent, is also related to the concepts for two piece swage type fasteners such as those illustrated in U.S. Pat. No. 3,915,053 to J. Ruhl, issued Oct. 28, 1975. Since the concepts of both the '096 and '053 patents are fully described therein a detailed description of these concepts is omitted for purposes of brevity and simplicity.
Swage type fasteners are used for a variety of different applications including the joining of workpieces of various types of materials with a predetermined clamp force. In aircraft applications for example the workpieces can be of a lightweight metal such as aluminum and/or of a lightweight plastic, composite material. The composite structure for example can be of graphite-epoxy materials or other similar type of plastic materials.
Such plastic or composite materials generally have a lower impact strength than metal workpieces and hence such workpieces when clamped by fasteners are subject to localized delamination or crushing under excessive loading and pressure in a direction along the Z axis or normal to their end surfaces engaged by the fasteners. This is especially true where there is a slight space or gap between workpieces being secured. In this regard it is common in aerospace applications to have an inner workpiece of a composite material secured to an outer workpiece of a metallic material such as aluminum. Here thin resinous coatings or shims are usually applied to the inner composite workpiece between the mating surfaces to compensate for surface irregularities but which can still result in slight residual gaps. Upon securing the workpieces together with fasteners the composite workpiece may deflect at the area of gap proximate to the fasteners which may cause the localized delamination or crushing. To inhibit this the clamp loads for such plastic or composite workpieces are frequently distributed over a relatively larger area by use of collars with enlarged flanges for engaging the surface of the workpieces. Even here, however, for pull type fasteners, extra care must be taken to avoid damage to the plastic or composite material upon pin break in severing the pintail section. In this regard, it has been found that while pin break will occur at a predictable design load, a substantially higher load can be imposed on the workpieces resulting from a shock load occurring from pin break. The shock load, frequently referred to as a "ghost load", is caused by the release of stored energy in the fastener and fastened joint upon pin break which stored energy is developed by the relative axial forces which are applied for swage and pin break. The higher magnitude of shock or ghost load can result in localized delamination or crushing of the composite workpiece even though the magnitude of the swage load or pin break load would not. As noted this is most likely to occur where a gap, even though slight, is present between the inner composite workpiece and the outer metallic workpiece in the area of the fastener. Thus the pin break load results in a "ghost load" of even greater magnitude. The magnitude of the pin break load, however, is dictated to a great extent by the magnitude of the relative axial force required to complete the swaging of the collar. In other words, the collar must be fully swaged at a first relative axial force before the relative axial force is increased by a predetermined amount to a higher magnitude to sever the pintail. In this regard the load required to sever the pintail is typically around 10% greater than the swage load.
With the optimized lightweight fasteners of the '096 patent, weight is minimized in part by the use of a lightweight collar. However, in order to provide the desired high strength, the lightweight collar is required to be swaged into the lock grooves of the pin with a relatively high predetermined level of overpacking. Overpacking or overfill is provided by use of a collar with the swageable portion having a volume which is a preselected amount greater than the available volume between the swage cavity of the installation tool and the lock grooves. Conventional, non-optimized fasteners are constructed to have a relatively low level of overpacking. However, in the existing optimized fastener, while the swageable portion of the collar has been reduced in volume, the magnitude of overpacking is increased by modification of the other components of the system. This requires substantially higher swage loads than for the conventional, non-optimized fasteners. In the present invention it has been discovered that by providing the swageable portion of the collar with a minimized or slight reduction in volume, and hence minimization or slight reduction in the amount of overpacking, and with a unique external contour, the swage loads can be significantly reduced with essentially no loss of performance compared to existing, optimized fasteners constructed as shown in the '096 patent. At the same time the lower swage load permits there to be a substantial reduction in the magnitude of the load required to sever the pintail. The result is a substantial reduction in the shock load or "ghost load" resulting from pin break. Another advantage is the ability to use a lighter weight pull type installation tool since the installation loads are substantially reduced. Even, however, if the same sized tool is used the lower installation loads will contribute to longer life of the tool components. This latter advantage of reduced swage load also permits the use of lighter weight squeeze type installation tools for stump type swage fasteners and/or provides increased life of the larger existing tools. In addition, for pull type fasteners, there will be a reduction in the reaction force felt by the operator.
With pull type fasteners, the size of the pintail section with pull grooves can be reduced resulting in a saving in material cost which can be significant especially when the pins are constructed of relatively expensive materials such as titanium. In this regard the modified design of the collar results in a slight reduction in material and final weight of the collar and thus of the installed fastener. Also while the fastener system of the present invention has the advantages noted when used to secure workpieces of a composite material, although less critical re installation and/or shock load damage for metal workpieces, it also provides certain ones of the advantages for use in securing workpieces of metal. For example lighter weight tools could be employed; tool life could be increased and, for pull type fasteners, the reaction load felt by the operator at pin break could be reduced and the noise at pin break would be reduced. At the same time there still would be a savings in material costs.
Therefore it is an object of the present invention to provide a lightweight, swage fastener of a construction for installation at a reduced swage load while providing a fastened joint with substantially the same strength as comparable existing lightweight swage fasteners requiring a higher swage load.
It is another object of the present invention to provide a lightweight pull type swage fastener with a severable pintail section which can be installed with a reduced swage load and with a reduced load for severance of the pintail section while providing a fastened joint of substantially the same strength as comparable existing lightweight swage fasteners requiring higher swage and severance loads.
It is also an object of the present invention to provide a lightweight, pull type swage fastener for securing workpieces made of plastic, composite materials and which can be installed with a reduced swage load while providing a fastened joint of substantially the same strength as comparable existing lightweight swage fasteners requiring a higher swage load.
It is another object of the present invention to provide a lightweight, pull type swage fastener with a severable pintail section for securing workpieces made of plastic, composite materials and which can be installed with a reduced swage load and a reduced load for severance of the pintail section resulting in reduced shock or ghost loads at severance whereby damage to the composite workpieces is inhibited while providing a fastened joint of substantially the same strength as comparable existing lightweight swage fasteners requiring a higher swage load.
It is still another object of the present invention to provide a lightweight pull type swage fastener with a severable pintail section and adapted to be installed with a reduced swage load and a reduced load for severance of the pintail section resulting in reduced shock or ghost loads while providing a fastened joint of substantially the same strength as comparable existing lightweight, swage fasteners requiring higher swage and severance loads.
It is another object of the present invention to provide a lightweight pull type swage fastener with a severable pintail section and adapted to be installed at a reduced swage load and a reduced load for severance of the pintail section with the construction of the pin and of the collar each providing a reduction in material.
Therefore, it is a general object of the present invention to provide an improved, lightweight fastening system including a swage type fastener of a construction such that it can be installed at a reduced swage load and, if of a pull type with a severable pintail section, it can be installed with a reduced load for severing the pintail section.
While the present invention is described primarily for a pull type fastener with a severable pintail section it should be understood that the low swage load construction can also be advantageously used with stump type fasteners such as shown in FIGS. 1-4 of the '096 patent. In this regard it should be understood that the low swage load construction of the present invention can be advantageously used with pull type swage fasteners without a severable pintail section in systems such as those shown and described in U.S. Pat. 5,315,755 to Fulbright and Smith issued May 31, 1994.
Other objects, features and advantages of the present invention will become apparent from the subsequent description and the appended claims, taken in conjunction with the accompanying drawings, in which: